1. Field of the Invention
The present invention relates to a polyester for a fiber or film, which is excellent in color and shapeability, such as spinnability or drawability, and also excellent in mechanical porperties such as strength and Young's modulus, and a process for the preparation thereof.
2. Description of the Related Art
Polyesters, especially polyethylene terephthalate, are used in various fields particularly as fibers and films, due to their many excellent properties.
These polyesters are ordinarily prepared by forming a glycol ester of terephthalic acid and/or a low polymer thereof by subjecting terephthalic acid and ethylene glycol to an esterification reaction, subjecting a dialkyl ester of terephthalic acid and ethylene glycol to an ester-exchange reaction or reacting terephthalic acid with ethylene oxide, and polycondensing the product by heating under reduced pressure until the degree of polymerization is elevated to a predetermined level.
A polyester prepared according to the above-mentioned process is extruded in the molten state through a spinning nozzle or a slit to form a fiber or film. The formed fiber or film is put into practical use. Furthermore, there is broadly adopted a method in which a preoriented yarn (POY) obtained by melt-spinning a polyester at a high speed exceeding 2,000 m/min is subjected to draw false twisting texturing.
Recently, there has been proposed a process in which a polyester is melt-spun at a high speed exceeding 5,000 m/min. A polyester fiber having sufficient properties for practical use is thus obtained only through the spinning step (see, for example, U.S. Pat. No. 4,415,726 to H. Tanji et al).
However, since an increase of the spinning speed, especially beyond 5,000 m/min, results in an increase of single filament breakage and yarn breakage, the obtained fiber has many defects, such as fluff, and the adaptability to subsequent processing operations is drastically degraded. This tendency becomes conspicuous as the spinning speed is increased, the single filament denier is reduced, and the filament number is increased. Accordingly, spinning at a speed exceeding 6,000 m/min is very difficult in practice.
In the field of fibers, along with the increasingly sophisticated treatments for improving the productivity and imparting various functional characteristics at the post-processing, spinning, and other steps, it has become strongly desired to improve the properties, especially the mechanical properties such as strength and Young's modulus, in both filaments and staple fibers of polyesters.
In the so-called direct yarn-forming process in which a polyester yarn ready for practical use is obtained by just the spinning step, it is necessary to increase the winding speed to at least 5,000 m/min. Even if the winding speed is increased to such a high level, the mechanical properties, such as strength and Young's modulus, of the polyester yarn obtained by the direct yarn-forming process are still lower than those of ordinary drawn yarns.
Moreover, at this high-speed spinning step, yarn breakage is frequently caused. Therefore, the process can not be realized effectively on an industrial scale.
One of us made research with a view to eliminating the foregoing disadvantages. As a result, it was found that a polyester formed by adding 0.01 to 1.5 mole % of p-hydroxybenzoic acid (often referred to as "POBA" hereinafter) to a bifunctional carboxylic acid or an ester-forming derivative thereof has a good color and, when this polyester is subjected to high-speed spinning, yarn breakage can be reduced at the spinning step and the strength and Young's modulus of the obtained fiber are highly improved. This process was already proposed in the specifications of Japanese Patent Applications No. 55-107753 and No. 55-116978.
A polyester formed by adding a small amount of POBA is excellent in adaptability to high-speed spinning over ordinary polyesters and the mechanical properties of the obtained fiber are improved. However, the mechanical properties are still insufficient from a practical viewpoint. It is desired to further improve the mechanical properties.